1. Field of the Invention
The invention relates to display method and apparatus for displaying information by projecting a light onto an external projection plane.
2. Related Background Art
Hitherto, various kinds of projector application equipment have been known as display apparatuses for displaying information by projecting a light onto a projection plane. Among them, as a display apparatus for projecting an image from the front side of the projection plane, an auto focusing type projector has been realized. As a display apparatus for projecting an image from the rear side of the projection plane, a television of the rear projection type has been realized.
A mechanism for adjusting a picture quality of the projection plane in each of those display apparatuses will now be described.
The auto-focusing type projector outputs an image by projecting the light onto the external projection plane in a manner similar to the overhead projector (OHP). An image output system is constructed by a plurality of optical lenses and can adjust a focal distance in accordance with a distance to the projection plane. The focal distance is adjusted by changing the distance between the projection plane and the lens by a built-in motor. The distance between the projection plane and the optical system in such an apparatus is generally detected by measuring a distance in an optical axis direction of the projector through a lens system. Its mechanism will now be described hereinbelow.
The measuring mechanism of the distance in the above display apparatus comprises: a light emitting device for outputting a reference light of an infrared wavelength in the direction of the optical axis of the projector; and a photosensitive device for detecting a reflected light through a lens system. The photosensitive device is a surface photosensing type device (SPD) whose output changes in dependence on a photosensing area. In the above construction, in the case where focal distances of the projection plane and the lens system are coincident, the photosensing area of the photosensitive device is minimum. When the focal distance is improper, the photosensing area of the photosensitive device is widened and detected. Therefore, a processing system of the distance detecting mechanism feedback controls the lens system motor on the basis of an output of the photosensitive device and corrects and drives the optical system, thereby adjusting so as to obtain a correct focal distance.
The television of the rear projector type will now be described. According to the display apparatus, by enlargedly projecting a television image onto a projection film plane provided in the front portion of the apparatus, a large projection display of 50 to 100 inches is realized.
A construction in the apparatus will now be described. To reduce the installing area, an optical path is changed by using a mirror, a prism, and the like. Thus, the optical axis of the light entering the projection film plane has an angle and a distortion occurs on the display screen. To prevent such a problem, the distortion is corrected in the optical system. In the above construction, since the positional relation between the projection film plane and the optical system is fixed, a distortion of the same state always occurs. In the display apparatus, therefore, a method of correcting the distortion by devising the shapes of the lens and prism is realized. That is, by setting magnifications and angles of the lens and prism in accordance with the optical path of the apparatus, the above distortion is corrected.
Further, a zoom lens of a camera is known as an apparatus which can change an image magnification. In such an apparatus, a picture plane can be arbitrarily enlarged or reduced by the motion of a built-in motor or a manual operation. A change in image magnification is realized by changing intervals among a plurality of lenses constructing the zoom lens.
In the above conventional apparatuses, however, when the distance from the projection plane changes, there is a problem such that when the direction of a normal line of the projection plane and the direction of the optical axis of the output light are deviated, a distortion occurs in the image. Namely, in the general display apparatus, for example, sizes of characters in the upper and lower portions of the display screen differ or a figure is distorted due to the distortion of the image as mentioned above. A construction to correct such a distortion is not provided in the conventional apparatuses.